webm2pes: Make WritePesPacket() a public method.

WritePesPacket() now takes a VpxFrame as input and a
PacketDataBuffer* as an output  parameter, and avoids
using members entirely. This facilitates deeper testing
of Webm2Pes without requiring friendship with test
classes and methods.

Change-Id: I076db8900cc1a5c864b54f5fe3403fb05b4bd835
2 files changed